The invention relates to a plasma picture screen provided with a front plate comprising a glass plate on which a dielectric layer and a protective layer are provided, with a carrier plate provided with a phosphor layer comprising a red and a blue phosphor as well as a green Tb3+-activated phosphor, with a ribbed structure which subdivides the space between the front plate and the carrier plate into gas-filled plasma cells, and with one or several electrode arrays on the front plate and the carrier plate for generating corona discharges in the plasma cells.
Plasma picture screens render possible color pictures of high resolution, large screen diameter, and have a compact construction. A plasma picture screen comprises a hermetically closed glass cell which is filled with a gas, with electrodes in a grid arrangement. The application of a voltage causes a gas discharge which generates light in the ultraviolet range (145 to 185 nm). This light can be converted into visible light by phosphors and be emitted through the front plate of the glass cell to the viewer.
Phosphors which are particularly efficient under vacuum UV excitation are used for plasma picture screens. Frequently used green-emitting phosphors are, for example, Zn2SiO4:Mn (ZSM) and BaAl12O19:Mn (BAL). Both materials show a saturated green emission color with a high y-value of y greater than 0.7. A disadvantage of both materials is their comparatively long decay time t1/10, for example 30 ms for Zn2SiO4 with 2.5% Mn. The cause of this is that the transition 4T1xe2x86x926A1 relevant for the emission of the light is spin-forbidden. In addition, the decay time t1/10 and the color point of an Mn2+-activated phosphor are strongly dependent on the Mn2+ concentration. A further disadvantage is the sensitivity of Mn2+ to an oxidation to Mn3+ or Mn4+, which reduces the stability of the phosphors.
By contrast, Tb3+-activated phosphors are temperature stable and photostable because Tb3+ does not readily oxidize to Tb4+. A further advantage of these phosphors over Mn2+-activated phosphors is their shorter decay time t1/10, which lies between 2 and 10 ms, depending on the host lattice.
U.S. Pat. No. 6,004,481 accordingly describes a green-emitting Tb3+-activated phosphor for use in plasma picture screens which has the composition (Y1xe2x88x92xxe2x88x92yxe2x88x92zGdxTbyCez)BO3, with 0.0 less than x less than 0.2, 0.01 less than y less than 0.1, and 0.0 less than z less than 0.1.
A major disadvantage of Tb3+-activated phosphors is their yellowish-green color point, which has a low y-value of y less than 0.62.
The invention has for its object to provide a plasma picture screen with a Tb3+-activated phosphor whose green pixels supply light with an improved color point.
This object is achieved by means of a plasma picture screen provided with a front plate comprising a glass plate on which a dielectric layer and a protective layer are provided, with a carrier plate provided with a phosphor layer comprising a red and a blue phosphor as well as a green Tb3+-activated phosphor, with a ribbed structure which subdivides the space between the front plate and the carrier plate into gas-filled plasma cells, and with one or several electrode arrays on the front plate and the carrier plate for generating corona discharges in the plasma cells, and with a green color filter layer.
Apart from a strong emission of light with a wavelength between 540 and 550 nm, Tb3+-activated phosphors also have emission bands, though substantially weaker, in the yellow and red spectral ranges. The intensity of these emission bands can be reduced by means of a green color filter layer, and the y-values of the color points of the Tb3+-activated phosphors can be raised thereby. Green color filter layers absorb strongly above 580 nm, so that also the intensity of the emission lines of the neon, which lie in this spectral range and which reduce the color saturation of green- and blue-emitting phosphors, is reduced.
It is preferred that the green color filter layer lies between the dielectric layer and the protective layer.
In this case the color filter layer can be provided on a plane surface, and the layer thickness of the color filter layer will not vary in dependence on the various regions of the front plate.
It is particularly strongly preferred that the green color filter layer lies in a structured manner opposite the regions of the phosphor layer with the green Tb 3+-activated phosphor.
In this case, only the undesired spectral ranges of the green light emission are absorbed by the green color filter layer.
It is in addition preferred that the color filter layer comprises copper phthalocyanine or a derivative of copper phthalocyanine.
Copper phthalocyanine or a derivative of copper phthalocyanine has a high color purity and a transmission maximum at the wavelength of the light emitted by the Tb3+-activated phosphors.
It is furthermore preferred that the green Tb3+-activated phosphor is chosen from the group (YxGd1xe2x88x92x)BO3:Tb (0xe2x89xa6xxe2x89xa61), LaPO4:Tb, (YxGd1xe2x88x92x) 3Al5O12:Tb (0xe2x89xa6xxe2x89xa61), CeMgAl11O19:Tb, GdMgB5O10:Ce,Tb, (YxGd1xe2x88x92x)2SiO5:Tb (0xe2x89xa6xxe2x89xa61), (InxGd1xe2x88x92x)BO3:Tb (0xe2x89xa6xxe2x89xa61), Gd2O2S:Tb, LaOBr:Tb, LaOCl:Tb and LaPO4:Ce,Tb.
These Tb3+-activated phosphors are particularly efficient green-emitting phosphors when excited with VUV light.
It is advantageous that an additional red color filter layer lies in a structured manner opposite the regions of the phosphor layer with a red phosphor.
It is also advantageous that an additional blue color filter layer lies in a structured manner opposite the regions of the phosphor layer with a blue phosphor.
An additional red or blue or red and blue color filter layer enhances the LCP (Luminance Contrast Performance) value of the plasma picture screen as a whole.